Why can't I animate custom properties (aka CSS variables)?
See this animation:
- The golden div has an animation where a custom property is animated
(@keyframes roll-o-1animates--o).
This animates in steps. - The silver div has an animation where a normal property is animated
(@keyframes roll-o-2animatesleft).
This animates continuously.
Why doesn't the golden div animate smoothly?
Is there any workaround which also uses variables?
#one {
width: 50px;
height: 50px;
background-color: gold;
--o: 0;
animation: roll-o-1 2s infinite alternate ease-in-out both;
position: relative;
left: calc(var(--o) * 1px);
}
@keyframes roll-o-1 {
0% {
--o: 0;
}
50% {
--o: 50;
}
100% {
--o: 100;
}
}
#two {
width: 50px;
height: 50px;
background-color: silver;
--o: 0;
animation: roll-o-2 2s infinite alternate ease-in-out both;
position: relative;
}
@keyframes roll-o-2 {
0% {
left: 0px;
}
50% {
left: 50px;
}
100% {
left: 100px;
}
}<div id="one"></div>
<br>
<div id="two"></div>Solution 1:
When this question was asked, it wasn't possible to animate custom properties, as @temani afif correctly pointed out -
since the UA has no way to interpret their contents
Since then, CSS Houdini have put together the CSS Properties and Values API specification
This specification extends [css-variables], allowing the registration of properties that have a value type, an initial value, and a defined inheritance behaviour, via two methods:
A JS API, the registerProperty() method
A CSS at-rule, the @property rule
So now that you can register your own custom properties - including the type of the custom property - animating the custom property becomes possible.
To register the custom property via CSS - use the @property rule
@property --o {
syntax: "<number>";
inherits: false;
initial-value: 0;
}
#one {
width: 50px;
height: 50px;
background-color: gold;
--o: 0;
animation: roll-o-1 2s infinite alternate ease-in-out both;
position: relative;
left: calc(var(--o) * 1px);
}
@keyframes roll-o-1 {
0% {
--o: 0;
}
50% {
--o: 50;
}
100% {
--o: 100;
}
}
#two {
width: 50px;
height: 50px;
background-color: silver;
animation: roll-o-2 2s infinite alternate ease-in-out both;
position: relative;
}
@keyframes roll-o-2 {
0% {
left: 0px;
}
50% {
left: 50px;
}
100% {
left: 100px;
}
}
@property --o {
syntax: "<number>";
inherits: false;
initial-value: 0;
}<div id="one"></div>
<br>
<div id="two"></div>To register the property via javascript - use the CSS.registerProperty() method:
CSS.registerProperty({
name: "--o",
syntax: "<number>",
initialValue: 0,
inherits: "false"
});
CSS.registerProperty({
name: "--o",
syntax: "<number>",
initialValue: 0,
inherits: "false"
});#one {
width: 50px;
height: 50px;
background-color: gold;
--o: 0;
animation: roll-o-1 2s infinite alternate ease-in-out both;
position: relative;
left: calc(var(--o) * 1px);
}
@keyframes roll-o-1 {
0% {
--o: 0;
}
50% {
--o: 50;
}
100% {
--o: 100;
}
}
#two {
width: 50px;
height: 50px;
background-color: silver;
animation: roll-o-2 2s infinite alternate ease-in-out both;
position: relative;
}
@keyframes roll-o-2 {
0% {
left: 0px;
}
50% {
left: 50px;
}
100% {
left: 100px;
}
}<div id="one"></div>
<br>
<div id="two"></div>NB
Browser support is currently limited to chrome (v78+ for registerProperty(), v85+ for @property) edge and opera
Solution 2:
From the specification:
Animatable: no
Then
Notably, they can even be transitioned or animated, but since the UA has no way to interpret their contents, they always use the "flips at 50%" behavior that is used for any other pair of values that can’t be intelligently interpolated. However, any custom property used in a @keyframes rule becomes animation-tainted, which affects how it is treated when referred to via the var() function in an animation property.
So basically, you can have transition and animation on property where their value are defined with a custom property but you cannot do it for the custom property.
Notice the difference in the below examples where we may think that both animation are the same but no. The browser know how to animate left but not how to animate the custom property used by left (that can also be used anywhere)
#one {
width: 50px;
height: 50px;
background-color: gold;
animation: roll-o-1 2s infinite alternate ease-in-out both;
position: relative;
left: calc(var(--o) * 1px);
}
@keyframes roll-o-1 {
0% {
--o: 0;
}
50% {
--o: 50;
}
100% {
--o: 100;
}
}
#two {
width: 50px;
height: 50px;
background-color: silver;
--o: 1;
animation: roll-o-2 2s infinite alternate ease-in-out both;
position: relative;
}
@keyframes roll-o-2 {
0% {
left: calc(var(--o) * 1px);
}
50% {
left: calc(var(--o) * 50px);
}
100% {
left: calc(var(--o) * 100px);
}
}<div id="one"></div>
<br>
<div id="two"></div>Another example using transition:
.box {
--c:red;
background:var(--c);
height:200px;
transition:1s;
}
.box:hover {
--c:blue;
}<div class="box"></div>We have a transition but not for the custom property. It's for the backgroud because in the :hover state we are evaluating the value again thus the background will change and the transition will happen.
For the animation, even if you define the left property within the keyframes, you won't have an animation:
#one {
width: 50px;
height: 50px;
background-color: gold;
animation: roll-o-1 2s infinite alternate ease-in-out both;
position: relative;
left: calc(var(--o) * 1px);
}
@keyframes roll-o-1 {
0% {
--o: 0;
left: calc(var(--o) * 1px);
}
50% {
--o: 50;
left: calc(var(--o) * 1px);
}
100% {
--o: 100;
left: calc(var(--o) * 1px);
}
}
#two {
width: 50px;
height: 50px;
background-color: silver;
--o: 1;
animation: roll-o-2 2s infinite alternate ease-in-out both;
position: relative;
}
@keyframes roll-o-2 {
0% {
left: calc(var(--o) * 1px);
}
50% {
left: calc(var(--o) * 50px);
}
100% {
left: calc(var(--o) * 100px);
}
}<div id="one"></div>
<br>
<div id="two"></div>